No rear brakes problem... proportioning valve question

[Zone47]

My truck is a 68 C10 - drum/drum with no booster.

I thought the problem was the master cylinder, so I bought a new one today, gave it a good bench bleed and install, still no rears. There is no fluid at all out of the bleed screws in the rear. Could it be the proportioning or routing block?

I ohmed the switch (out of the bottom) to ground but it's not shorted. I removed it but it's just a flathead screw head on the inside with a couple of o-rings. I don't feel any notch on the inside. It's a very strange setup.

So it looks like the faults are either the proportioning valve, the rear flex line, or the wheel cylinders? Which sounds like the most likely culprit in this case?

This pic is with the old master cylinder>